Epitaxy is the growth of a crystalline layer, typically from a seed layer. Selective epitaxial growth (SEG) is one type of epitaxy adapted for growth of a single crystalline layer on only a selected portion(s) of a substrate called seed window areas in a channel hole. The surface of the substrate can be selectively exposed by removing an intervening protective layer to form a seed window area. With the scaling down of device dimensions, such as for three-dimensional (3D) NAND flash memory devices, reduced seed window area dimensions and SEG pre-cleaning processes can cause non-uniformity in the substrate surface. In particular, SEG pre-cleaning processes can greatly affect the epitaxial growth rate and thickness of an epitaxial layer in the seed window area, as well as corrode the channel hole.
Significant substrate lattice damage can occur when a channel hole is formed in a patterned device. Normally hydrogen chloride (HCl) gas will be used to remove a damaged substrate. However, it is hard to control the HCl flow. If the HCl gas flow is too high, an undamaged substrate will also be removed and the channel hole CD (Critical Dimension) will be enlarged and the device performance may shift. If the HCl gas flow is too low, unwanted growth of silicon will occur on the sidewalls of the channel hole and block the channel hole. Both conditions can affect overall product yield. Further, the thermal budget (i.e., maximum temperature) of subsequent processes for repair of a damaged substrate lattice is limited. The non-uniformity in substrate surface makes device processing difficult to control and can adversely affect device performance.